Electromechanical transducer



Aug. 19, 1952 w, MASON 2,607,858

ELECTROMECHANICAL TRANSDUCER Filed June 19, 1948 H 1 i i i i 20 m SIGNAL SOURCE .//9

/NVENTOR W I? MASON ATTORNEY Patented Aug. 19, 1952 UNITED STATES ELECTRQMECHANICAL TRANSDUCER Warren P. Mason, West Orange, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application June 19, 1948, Serial No. 34,040

4. Claims. (Cl. 179 -110) This invention relates to electromechanical transducers and more particularly to electroacoustic translating devices, such as telephone receivers. I

Objects of this invention are to simplify the structure and to increase the efficiency of electromechanical transducers and to obtain a high output level for telephone receivers.

The invention is predicated in part upon the discovery that a body of polycrystalline barium titanate (BaTiOslcan becaused tovibrate in the thickness mode in response to an applied alternating current signal, with a high electromechanical coupling and that this effectmay be utilized practically to translate electrical signals into usable mechanical or acoustic energy.

Barium titanate is a readily moldable and mechanically workable ceramic having a dielectric constant somewhat in excess of 2000. If a disc thereof is subjectedtoa small alternating-current signal applied between its faces, it acts simply as a condenser. 11, however, the disc is polarized in the direction normal to its faces, as by theapplication of a high direct-current voltage between these faces, and a small alternating-current. signal is then applied, either superimposed upon the direct-current voltage or after the latter has been removed, mechanical resonances are produced. The effects are electrostrictive and the most prominent is associated with vibration in the thickness longitudi- The electromechanical coupling facnal mode. tor for this mode may be of the order of 0.5.

The voltage for applying the direct-current bias or inducing the remanent polarization should belarge, of the order of 5 to 30 kilovolts per centimeter, and itsmagnitude affects the coupling factor. Generally,.thehigher'the polarizing or biasing voltage the higher the con pling factor, although the rate of increase of coupling factor with increasing voltage decreases as the voltage increases. Also the electro' strictive constant for the thickness mode increases as the applied direct current voltage increases.

In one illustrative embodiment of this inven tion, an electromechanical transducer comprises a ring or cylinder of barium titanate which is polarized in the direction of its thickness, that is radially, by a strong direct-current voltage and is excited to vibrate radially by applying a small alternating-current signal thereto in the direction of the polarization. As the thickness of the ring or cylinder increases and decreases, in response to the applied alternating-current signal, the ring expands and contracts accordingly.

The ring or cylinder may be utilized to drive an operating element, such as an acoustic diaphragm. In one embodiment the .ring or cylinder is coupled to the diaphragm by a spider or other connector in such manner as to efiect substantially bodily vibration of the diaphragm parallel to the axis of the ring or cylinder. The coupling or connector advantageously is constructed to produce a large transformation ratio, whereby a desirable high air load impedance for the diaphragm is realized.

The parameters of the vibrating system, including the ring or cylinder and the diaphragm,

- are correlated so that the system is resonant at a ,preassigned frequency, for example at the highest frequency in the range to be translated bythe device.

The invention and the features thereof will be understood more clearly and fully from the following detailed'description with reference to the accompanying drawing in which:

Fig. 1 is in part a sectional view and in part a block diagram of a telephone receiver illustrative of one embodiment of this invention;

Fig. 2 is a side view of the diaphragm and driving ring unit included in the receiver shown in Fig. 1, a portion of the unit being broken away and the nature of the diaphragm motion being indicated; and v p I Fig.3 is a sectional view of a telephone receiver-illustrative of another embodiment of this invention, the casing beingomitted for the sake of simplicity of illustration.

Referring now to the drawing, the electroacoustic transducer illustrated in Fig. 1 comprises a casing E0 to which a cap or an earpiece I 5 having a plurality of apertures l2 therein is threaded. Within the casing and seated upon a resilient, e. g; soft or sponge rubber annulus l3 therein, is a ring or short cylinder Hl of barium titanate having electrodes i 5 and I6, for example metal coatings such as silver pasteupon the in ner and outer faces thereof. Adiaphragm, for

example of a lightweight metal such as Duralumin, is fixed to .one end of the ring or cylinder M, the diaphragm having a central, dished, substantially bodily yibratile portion I1 and a surrounding portion. l8 the periphery of which is secured to the ring I 4, as by cementing.

Electrical connection may be established between an input signal source l9 and the electrodes l5 and I6 by way of conductors 20. The ring may be polarized in the direction of its thickness, that is radially, by a high voltage direct-current source 2! connected across the conductors 20 through a switch 22.

In response to an alternating-current signal applied from the source I9, the ring [4, by electrostrictive action, is excited to vibrate. That is to say, the ring will vary in thickness cyclically and the thickness vibration will excite vibration of the ring in radial modes. This vibration is translated into vibrations of the diaphragm H. The nature of the diaphragm'displacement ,is illustrated in Fig. 2 wherein the position of the diaphragm for expanded and contracted conditions of the ring 14 as it vibrates are indicated by the dotted lines HA and HE respectively. The motion of the central portion ll of the diaphragm is essentially piston-like; the portion l 8 of the diaphragm acting as a lever arm coupling the ring l4 and the diaphragm portion I! whereby the vibration of the ring [4 is transformed into larger amplitude vibration of the diaphragm portion II.

In the embodiment of the invention illustrated in Fig. 3, the barium titanate ring I4 is coupled mechanically to the apex of a dished or conical diaphragm I70 by a pair of lightweight, stiff armed spider'members 23 which are connected at their centers by a rigid post 24. Expansion and contraction of the ring 14 in response to an alternating-current signal superimposed upon the strong direct-current polarization is transformed into bodily vibration of the diaphragm. Inasmuch as the spider arms are stifi, a high transforming ratio is obtained, that is, a transformation of sidewise motion of the ring into up and down motion of the diaphragm in the ratio of r/y is realized. .This ratio may readily be made of the order of 50 to 100 so that the air impedance of the receiver may be transformed up in the ratio-of the order of 2,500 to 10,0.00.

In the design of a transducer such as those illustrated in Figs. 1 and 3 certain parametral magnitudes and relationships are advantageous. For example, advantageously'the barium titanate ring is made thin, for example of the order of 0.03 centimeter in a telephone receiver, whereby a low electrical impedance and low mechanical impedance with a consequent good match to the air load impedance on the diaphragm are obtained. Also, the use of a relatively low directcurrent biasing voltage is thus-enabled. The constants of the vibrating system should be correlated so that this systemincluding the barium titanate ring is resonant at about the highest frequency'to be translated by the device. For example, in a typical receiver, the ring I4 may be 3.0 centimeters in diameter and 0.03 centimeter thick radially and the direct-current bias may be 30,000 volts per centimeter whereby the highest thicknessmode resonance frequency of the ring is about 50,000 cycles. The mass of the ring plus the mass 'of the diaphragm in the device illustrated, in Fig. 1,or plus the mass of the ring and the spider in the device illustrated in Fig.3, are correlated with the stiifnesses of the vibrating elements in known Ways to produce resonance of the system atthe desired frequency.

In devices of the constructions described, a high electromechanical coupling is realized because of, the excitation of .the barium titanate in the thickness mode. Hence, high translation efiiciencies areattained. Furthermore, because of the large dielectric constant, in excess of 2,000, of the barium titanate, the electrical impedance of the devices is quite low and, therefore, the devices are suitable for use in place of conventional transducers of other than the crystal type.

Although the invention has been described with reference to telephone receivers, it may be utilized also in transmitters. Furthermore, although specific embodiments of the invention have been shown and described, it will be understood that they are but illustrative and that various modifications may be made therein without departing from the scope and spirit of this invention as defined in the appended claims.

What is claimed is:

1. Anelectroacoustic translating device comprising a radially polarized ring of barium titanate, means for exciting said ring to effect electrostrictive vibration thereof in the thickness mode, and a diaphragm coupled to said ring for actuation by vibration thereof.

2. An electroacoustic translating device comprising a ring of barium titanate, a diaphragm adjacent said ring and having its periphery connected thereto, means for polarizing said ring radially, and means for impressing an alternating-current signal radially upon said member.

3. An electroacoustic translating device comprising a circular cylinder of barium titanate, a diaphragm opposite one end of said cylinder and having a central portion and a surrounding portion connected to said one end of said cylinder, means for impressing a direct-current polarizing voltage upon and radially of said cylinder, and

means for applying an alternating-current signal REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date.

2,164,638 Broeze et a1. July 4, 1939 2,224,891 Wright Dec. 17, 1940 2,235,489 Rath Mar. 18, 1941 2,360,479 Detrick Oct. 1 7, 1944 2,363,409 ,Gibson Nov. 21, 1944' 2,402,515 .Wainer June 18, 1946 2,477,596 I Gravley Aug. 2,, 1949 2,486,560 Gray Nov. 1,1949 2,487,962 Arndt Nov. 15, 1949 OTHER REFERENCES Dielectric and Piezoelectric Properties of Barium Titanate, by S. Roberts, Physical Review,

vol. 71, No. 1'2,J u'ne 15,1947, pages 890-895. 

1. AN ELECTROACOUSTIC TRANSLATING DEVICE COMPRISING A RADIALLY POLARIZED RING A BARIUM TITANATE, MEANS FOR EXCITING SAID RING TO EFFECT ELECTROSTRICTIVE VIBRATION THEREOF IN THE THICKNESS MODE, AND A DIAPHRAGM COUPLED TO SAID RING FOR ACTUACTION BY VIBRATION THEREOF. 